# Find the mass of one element of a sample

• Chemistry

## Homework Statement:

A sample of potassium chromate, K2CrO4, is analyzed and found to contain exactly 0.67 grams of potassium. What is the mass of this sample of potassium chromate?

## Relevant Equations:

Formula mass = (2 x K) + (1 x Cr) + (4 x O)
1 mole of K2CrO4 has formula mass of ((2 x K) + (1 x Cr) + (4 x O)) g
=> ((2 x 39.0983) + (1 x 51.9961) + (4 x 15.9994)) g = 194.1903 g
we have 2 moles of K in 1 mole of K2CrO4 = (2x 39.0983) g = 78.1966 g
so for 1 mole of substance we have 78.1966 g
So how many moles of the substance do we have if our sample contains just 0.67 g of K
=> 1 mole x (0.67/78.1966) = 0.00856 moles
So the mass of K2CrO4 in 0.00856 moles is ( 0.00856 x194.1903) g
=> Mass of K2CrO4 of this sample is 1.66386 g
Now I was not sure of the number of significant figures required for the solution
since we were originally given the amount 0.67 g of K
I was unsure if this is to 2 or 3 significant figures so I gave the answer to both
I put down 1.66 g (to 3 sig fig) and 1.7 g (to 2 sig fig)
But both answers appear to be incorrect … can anyone help I am stuck ?

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etotheipi
2019 Award
As far as I can tell, your numbers are right. A more direct way to do it might have been to use the ratio $$\frac{2\times 0.39}{2\times0.39 + 52 + 4\times 16} = \frac{0.67}{M}$$

jim mcnamara
As far as I can tell, your numbers are right. A more direct way to do it might have been to use the ratio $$\frac{2\times 0.39}{2\times0.39 + 52 + 4\times 16} = \frac{0.67}{M}$$
Hi etotheipi,
Thank you for taking the time to look at my problem and for your advice. it has been a help to me to see the problem tackled in an alternative way. much appreciated.
Regards,
jackthehat

BillTre and etotheipi
epenguin
Homework Helper
Gold Member
You meant to write 39 not 0.39?